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A New Tilt on SETI

The planets in our Solar System rotate around the Sun more or less in a plane (the ecliptic) that is tilted some sixty degrees with relation to the galactic disk. It’s interesting to speculate that this could have ramifications in terms of the SETI hunt. Shmuel Nussinov (Tel Aviv University) considers the possibility that any extraterrestrial civilizations might try to contact us only after they had a fair idea we were here. And just as we are now trying, via Kepler and CoRoT, to track down small planets using the transit method, so too might extraterrestrials try to observe our transits, and having done so, to transmit a message.

Targeting habitable planets should optimize chances for a successful reception. From our end, a prudent SETI strategy might then be to home in on the ‘stripes’ of the sky within which our system’s planetary transits are detectable from other solar systems. As Nussinov writes:

The thickness of the galactic disc in our neighborhood is ∼ 150 parsecs. With the ecliptic at 60° relative to the disc the radial extent of the above slices where some eclipsing in the solar system is observable is typically ∼ 100 parsecs. This distance spikes at ∼ 10 K-parsecs towards the intersection of the ecliptic with the galactic plane.

Thus, if we consider only those stars (and prospective ITS’s thereabout) from which at any specific time eclipse by the inner planets can be seen, we restrict to 1.5%-2.5% of all candidates and to ∼ 7% if we use the broader +/- 3.4° stripe.

By ‘ITS’ Nussinov means Intelligent Technological Societies, cultures able to become aware of us through their own planet-finding technologies and thus more likely to transmit a signal in our direction. And as the author speculates, the notion might come into play with regard to the Fermi paradox. After all, if planetary transits are the primary detection methods at work around the galaxy, then the reason we may not be aware of extraterrestrials is that they simply haven’t found us yet:

This is due to the confluence of 1) our ecliptic plane being inclined by 60% to that of the galaxy—hiding us from most potential ITS’s which are actively searching; and 2) a noisy Sun surface further impeding discovery via the transit method.

Suppose Kepler flags fifty or so terrestrial planets, some of them in the habitable zone of their stars. Would interest in sending signals toward these planets result in actual transmissions? The answer is clearly yes, based upon what we’ve seen in recent years here on Earth, when signals have been transmitted to promote movies and snack foods. We’ve considered whether such messages are wise many times in these pages (search the site under ‘METI’) — my opposition to such transmissions is on record — but our culture show no sign of putting on the brakes. It’s interesting to speculate that an alien culture might act the same.

The paper is Nussinov, “Some Comments on Possible Preferred Directions for the SETI Search” (available online). I was sure I had run into this concept before and, after working on it subconsciously over night, finally recalled Richard Conn Henry (Johns Hopkins), who spoke on this topic at a recent AAS meeting. Here’s my post on that 2008 suggestion, and I’ll quote Henry from it:

“If those civilizations are out there — and we don’t know that they are — those that inhabit star systems that lie close to the plane of the Earth’s orbit around the sun will be the most motivated to send communications signals toward Earth, because those civilizations will surely have detected our annual transit across the face of the sun, telling them that Earth lies in a habitable zone, where liquid water is stable. Through spectroscopic analysis of our atmosphere, they will know that Earth likely bears life.”

This is clearly an idea that is coming into play. Henry notes the particular interest that Taurus and Sagittarius should have for this search, being intersections of the ecliptic with the galactic plane. Observatories like the Allen Telescope Array (ATA) may find this a profitable place to look.

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  • ljk March 15, 2009, 19:49

    Scientists Sometimes Need to Think Like Science Fiction Writers

    By Mike Brotherton

    March 14, 2009

    Not every scientist has an interest in science fiction or anything resembling the ability to think like a science fiction writer.

    I was reminded of this again today in regard to this story about how to focus SETI searches.

    Basically, the idea is that it is easier in principle to find and characterize Earth-like planets when you can see them eclipsing their system’s star. Therefore if we want to talk with ETIs, we should be targeting our searches in the plane of the ecliptic. “Plane of the ecliptic” is just a technical term for the directions in space that would see us eclipsing the Sun. The aliens living in star systems in the plane of the ecliptic, so the reasoning goes, would then be the ones most likely to have spotted us and be trying to communicate with us.

    Maybe a teeny-tiny bit more likely, but overall a dumb reason to restrict searches in my opinion. (Somehow the article I linked to talks about using the idea to “broaden” searches, but that’s just bad reasoning or writing, in my opinion. The only way to use the idea is to restrict searches.) Let me explain why I think the idea isn’t very good or logical, setting aside my scientist cap for my science fiction writer cap.

    Full article here:


  • Alexander L. Zatsev March 17, 2009, 4:53

    I guess, Shmuel Nussinov’s paper fits both for Sending (METI) and Searching for (SETI) IRMs (Interstellar Radio Messages).

    In 2006 in my paper:

    Messaging to Extra-Terrestrial Intelligence (METI)

    I already wrote about ecliptic as one of possible direction both for SETI and METI, see section 1 in above paper:

    “Where to send interstellar radio messages?

    It has become much easier to answer this question since 1995, when an outstanding discovery was made. Swiss astronomer Michael Mayor and graduate student Didier Queloz announced in that year the detection of the first planet orbiting another Sun-like star, 51 Pegasus. Subsequent discoveries of well over 100 other exoplanets have made it clear that planets are ordinary celestial objects, as widespread as stars and galaxies.

    In our Galaxy alone, with on the order of 100 billion stars, 1% of them are stars of solar or nearly solar types. Here, among this remarkable billion, it is plausible to select stars to which our interstellar radio messages can be addressees. We do not propose restricting our targets to only these stars, but they should be our main goal, defined by our present understanding, recognizing that the question of other life sites is not yet settled, and that there remains an opportunity for further creativity and research.

    Our present list of requirements for candidate stars includes the following characteristics:

    • Main sequence stars;

    • Constant luminosity;

    • Age in the range of 4 to 7 billion years;

    • Single stars of spectral classes close to that of the Sun are preferable;

    • Position in the sky close to “preferable directions” –

    near the ecliptic plane,

    in the direction of remarkable astronomical objects, toward the center or the anti-center of the Galaxy, etc.;

    • It is desirable that we fall in the direction of remarkable astronomical objects as viewed from There, so that They might find us in the course of Their usual astronomical observations;

    • In case of targets representing known planetary systems, it is desirable that orbits of these exoplanets have low eccentricity, as such planetary systems are more stable, and there is no significant temperature fluctuation interfering with the origin of life;

    • It is desirable to choose stars inside the “Belt of a Life” – that “hothouse” area of our Galaxy, where because of coincidence of speeds of movement of stars and spiral sleeves, conditions for origin and long development of a life are believed optimum.

    In due course, in the process of accumulating knowledge about the Cosmos, other criteria, and other locations than the stars addressed here, may emerge. For now, we propose concentrating on the above criteria.”

  • Robin Goodfellow March 17, 2009, 5:57

    Unfortunately, this line of reasoning is similar to that in The Fermi Paradox. It speculates based on knowledge of alien civilizations which we most certainly do not have.

    In many ways it’s similar to the equally premature speculation about extrasolar planets before we had discovered any. In the very early 1990s the prevailing thoughts were that extrasolar planets were probably pretty rare, perhaps exceedingly so. But this was biased by our ignorance and an improper appreciation for the vastness of the problem space that had remained almost completely unexplored.

  • Administrator March 18, 2009, 8:38

    Alexander Zaitsev wrote:

    It is desirable that we fall in the direction of remarkable astronomical objects as viewed from There, so that They might find us in the course of Their usual astronomical observations…

    Interesting! The idea seems to be in play in multiple venues, though your statement of it is the earliest I’ve seen.

  • John Wright March 28, 2009, 11:22

    Sending signals into space is like ringing a dinner ball.Everything on this planet eats something else, preditor verses pray. I imagine the cosmos isn`t much different just on a bigger scale.Hopefully SETI isn`t sending out our return address! Unless of coare the Aliens are lean without to much fat and easily domesticated.

  • ljk January 26, 2010, 14:21

    SETI founder Dr Frank Drake outlines ambitious plans

    Wired Science Jan. 25, 2010


    A radio observatory on the far side of the moon to eliminate Earth-based radio interference and gravitationalmicro lensing to view alien planets are among the projects for detecting extraterrestrial intelligence proposed by SETI pioneer Dr. Frank Drake….